Soldering components to a circuit board assembly typically leaves contaminants such as solder flux, resins, and ionic compounds. These contaminants must be removed if the circuit board assembly is to be sealed from the elements by a protective coating. The protective coating may not properly adhere to an assembly containing contaminants. The contaminants may also be removed for aesthetic purposes, to prevent corrosion, or to increase the assembly's insulation resistance.
The cleaning process to remove the contaminants is usually done with chlorofluorocarbon (CFC) based solvents in liquid or vapor form. These solvents are frequently an azeotrope of trichloro-trifiuoroethane and another solvent such as an alcohol, an example being methanol. One such solvent is a fluorinated hydrocarbon manufactured by E. I. DuPont de Nemours & Co. and marketed under the trade name FREON TMS.
The cleaning process is typically accomplished by placing the solvent in a container having heating elements at the bottom and condensing elements near the top. The solvent is then heated to its boiling temperature of 43.3.degree. C., producing a solvent vapor above the liquid solvent. The circuit board or electronic assembly to be cleaned is placed in the vapor. The electronic assembly, being at a lower temperature than the vapor, causes the vapor to condense on the assembly, thereby cleaning off the contaminants by dissolving or flushing off the contaminants.
The vapor that did not condense on the electronic assembly is retained in the cleaning apparatus by the condensing elements. These elements, typically operated at a temperature of 10.degree. C., cause the higher temperature vaporized solvent to condense back to liquid, returning to the bottom of the container to repeat the cycle. While the condensing elements in the container keep most of the solvent in the container, some of the solvent diffuses across the air-solvent interface and therefore escapes into the atmosphere. In addition, the assemblies being cleaned may retain small amounts of solvent on their surfaces after removal from the cleaning apparatus. Also, the spilling of solvent by workers refilling the cleaning apparatus can release the solvent into the atmosphere.
The solvent must be retained in the apparatus due to its expense and because of environmental concerns. In recent years, scientific studies have shown that CFCs have been depleting the protective ozone layer above the Earth, causing holes to form in this layer.
Alcohols such as isopropyl, methanol, or ethanol have been tried as an alternative to CFC based solvents. The assembly to be cleaned is initially immersed in and then sprayed with isopropyl alcohol, dissolving and/or washing off the contaminants. Alcohol, while it adequately removes ionic material from the assembly, may leave a white residue. Customers buying the assembly might get the impression that the assembly has not been cleaned or is defective in some way. There is a resulting need for an environmentally safe cleaning solvent that removes ionic material and does not leave a residue on the assembly being cleaned.